LMR systems are deployed by organizations requiring instant communication between geographically dispersed and mobile personnel. Typical users of LMR systems include police departments, fire departments, medical personnel, EMS and the military.
Current LMR systems can be configured to provide for radio communications between a site and subscriber units in the field. A subscriber unit may be a mobile unit or a portable unit. LMR systems can be as simple as two subscriber units communicating between themselves and a site over preset channels, or they can be complex consisting of hundreds of subscriber units and multiple sites.
LMR systems can be broadly divided into two classes: (1) trunking LMR systems; and (2) conventional LMR systems. A trunking system generally includes of one or more trunking sites and dispatch control centers. FIG. 1 illustrates a typical trunking LMR system 100 including a trunking site 104 and a dispatcher 108. The trunking site 104 includes a control channel 112 and one or more traffic channels (e.g., 116, 118). Typically a group of mobile users (e.g., 124, 128) creates a user group to communicate with each other and the dispatcher 108. In a trunking system 100 there can be multiple mobile users and multiple user groups. When a mobile user wants to contact other mobile users or a dispatcher in the same user group, the mobile user sends a request to a trunking site controller 132 through the control channel 112. The trunking site controller 132 contacts the other trunking sites interested in the same user group. The trunking site controller in each interested site allocates an available traffic channel. Once a channel is available, the mobile users in the user group is notified through the control channel and the mobile users move to the traffic channel to communicate with each other. Since a traffic channel is allocated dynamically on a per call basis, a trunking system provides higher utilization of a traffic channel. However, since a channel is not dedicated to a user group, a user may need to wait for a traffic channel when there is no available traffic channel.
The trunking LMR system 100 is generally suitable for high density areas where there is a likelihood of collision caused by multiple radios vying for a traffic channel. The control channel 112 is used to avoid collision by allocating available traffic channels to the radios.
FIG. 2 illustrates a conventional LMR system 200. Like the trunking system 100, the conventional LMR system 200 may include one or more conventional sites, although only one conventional site 204 and a dispatcher 208 is shown in FIG. 2. However, unlike the trunking site 104, the conventional site 204 does not include a control channel. The conventional site 204 includes a plurality of traffic channels, 216, 220, 224, each traffic channel being typically assigned to one or more user groups (e.g., 228, 232). The members of a user group may communicate with each other on the same traffic channel, thus allowing the users and the dispatcher to instantly communicate with each other without waiting for the system to allocate a traffic channel. However, the channel utilization is low in the conventional system 200 since multiple user groups may compete for channel usage on a particular traffic channel while a different traffic channel may be free to use.
In the conventional site 204, a mobile user presses a push to talk button and immediately starts talking. All mobile users and dispatchers in the same user group immediately hear the call. If multiple users key up at the same time, the calls are lost because of collision. Consequently, the conventional LMR system 200 is generally suitable in rural areas with low traffic density where in some cases only one channel may be available.
Thus, both the trunking LMR system 100 and the conventional LMR system 200 allow the mobile users to communicate via the traffic channel within their user groups. For example, the police radio 124 and the police radio 128 in the trunking LMR system 100 can communicate within their specific user group over a traffic channel assigned on a per call basis. Likewise, the police radio 228 and the ambulance radio 232 in the conventional LMR system 200 can communicate within their specific user groups over a traffic channel. However, if a police radio from a trunking LMR system needs to communicate with a police user group from a conventional LMR system, the dispatcher must patch the call to enable the two similar user groups to communicate. Since the dispatcher needs to patch the call to allow the police radio from the trunking LMR system to communicate with the police radio from the conventional LMR system, the reliability of such communication is degraded due to the reliance on the dispatcher. Also, the dispatchers generally use an analog 4-wire interface for patching the communication between the trunking LMR and conventional LMR systems, which is not scalable and provide limited flexibility of deployment.